This invention discloses a method and apparatus for the production of nitric oxide (NO) in controlled and accurate amounts, with low levels of impurities by controlling electric discharges between two electrodes in an oxygen nitrogen gas mixture.
Nitric oxide is known to have many applications in biological systems of both plants and animals.
In plants it is known that modifying the local atmospheric concentration of nitric oxide can stimulate a number of beneficial effects including, improved growth (U.S. Pat. No. 6,242,384), reduction in seed dormancy (Bethke, 2006 and Sarath, 2006), protection from fungal infections and disease (Lazar, 2008 and Hong, 2007) and preservation of cut flowers and fruit (U.S. Pat. Nos. 6,451,363 and 6,720,017).
In medical applications, gaseous nitric oxide administered to the patient is known to have multiple applications as disclosed in the following examples.
Anti-microbial: Nitric oxide has been demonstrated to reduce bacterial infections as shown during in-vitro testing (Ghaffari, 2006) and in clinical applications such as skin tissue infections (Ghaffari, 2007) and cystic fibrosis lung infections (Sagel, 2009).
Wound Healing: Nitric oxide has been demonstrated to improve healing times in both sterile and infected wounds (Shekhter, 2005).
Hemoglobinopathy: (U.S. Pat. No. 5,885,621) with application in sickle cell disease, where nitric oxide significantly reduced pain associated with vaso-occlusive crisis in sickle cell patients as compared to placebo (Head, 2010).
Selective pulmonary vasodilatation: (U.S. Pat. No. 5,485,827) with application in hypoxic respiratory failure of the newborn, where nitric oxide therapy selectively dilates the pulmonary vasculature and improves oxygenation, with no negative impact on systemic blood pressure (Clark, 2000).
Anti-inflammatory: (U.S. Pat. No. 6,656,452) with application in reducing ischemia reperfusion injury and the infarct size after myocardial infarction (Liu, 2007).
It is clear that with all these potential commercial applications that make use of the biological effects of nitric oxide, there needs to be an apparatus to deliver nitric oxide in an accurate and controlled amount that is reliable and efficient. One issue that has to be taken into account when considering a method of nitric oxide delivery is that of nitrogen dioxide (NO2) generation. Nitric oxide, when in the presence of oxygen, reacts to form nitrogen dioxide, which is am irritant to biological systems. To resolve this issue, there are two main approaches for generating and then controlling the delivery of nitric oxide to biological systems. A first approach is to produce nitric oxide from chemical precursors, for example, through the oxidation of ammonia and to store the nitric oxide with a diluent gas that does not contain oxygen (the gas used is normally nitrogen) in a high pressure cylinder. As the nitric oxide is needed, a delivery system (for example, a metering valve) controls the flow of nitric oxide gas from the cylinder to provide the amount of nitric oxide needed for any given application. The benefit of this approach is that it is relatively easy to control the amount of nitric oxide gas needed for a specific application and the purity of the nitric oxide can be ensured by a well controlled production process in a centralized production manufacturing location. The main problem with this approach is that the cylinders of compressed nitric oxide gas are large and heavy and are logistically difficult and expensive to ship from the centralized location to the site of application.
A second approach is to generate the nitric oxide gas in-situ from room air, using a controlled electric discharge to ionize the gas at a locally higher temperature to form a plasma, where oxygen and nitrogen in the air break down and reform to produce nitric oxide. This nitric oxide generating approach has the advantage that it does not have the logistical problems of the gas cylinder storage method. It is however, more difficult to accurately and controllably produce the required amounts of nitric oxide with the required purity.
Before providing a description of the invention an overview of background art will be described.
The use of electric discharges to produce nitric oxide in a plasma reaction has a long history, a good summary of which is included in U.S. Pat. No. 4,287,040 by Alamaro. This early prior art was focused on the bulk production of nitric oxide as an intermediary to the production of nitrogen based fertilizers and describes a process that is not concerned with the accuracy, purity and safety of the nitric oxide generated.
U.S. Pat. No. 5,396,882 (Zapol) was the first to disclose a system for producing nitric oxide by electric discharge for use in medicine. In this method, there is an electrically insulated reaction chamber where a high voltage circuit is used to induce an electric arc discharge between two electrodes that are separated by an air gap to produce nitric oxide. The patent discloses gas filters in the inlet conduit to the reaction chamber to remove liquid droplets or solid particles from entering the reaction chamber, and a soda lime filter in the outlet conduit of the reactor chamber for removing impurities such as nitrogen dioxide that may be formed in the plasma along with the nitric oxide. Also described is a gas analyzer such as a chemiluminescence analyzer for measuring the amount of nitric oxide produced. The high voltage circuit includes a step up transformer, which takes standard AC power of 110V and 60 Hz (230V and 50 Hz in Europe) in the primary coil, and steps up the voltage so that the peak voltage is sufficient to induce an electric are across the electrode air gap. There is a capacitor on the secondary side of the transformer, which is charged up to the breakdown voltage, and subsequently discharged across the gap when the breakdown voltage is reached. The current to the primary side of the transformer is regulated by an autotransformer (Variac), which controls the power to the capacitor and hence on to the electric arc discharge. The current from the capacitor is not controlled once break down voltage has occurred, and this results in an arc discharge which is quick and intense with a high current and high gas temperatures. The system describes producing nitric oxide continuously and controls the amount generated by controlling the current to the transformer, it also describes controlling the amount of diluting gas flow to provide the desired concentration.
There are a number of problems with this type of system which include: electric arc discharges cause high current at high temperatures which cause vaporization of the electrode material which leads to excessive wear of the electrodes. Electrode wear is a function of the intensity of the discharge across the electrodes, which translates into the higher the current the higher the electrode wear. The high temperature can also result in higher levels of nitrogen dioxide being formed, which is not desired in a number of biological applications. In addition, due to the electrode wear, the amount of nitric oxide generated in this system is not accurately predictable over periods of time and it requires a nitric oxide gas analyzer to ensure the expected amount of nitric oxide is being generated accurately. A gas analyzer adds expense, bulk and requires the user to calibrate it prior to use, which makes it undesirable for an optimum nitric oxide generation system. The patent discloses a filter for removing nitrogen dioxide from the nitric oxide gas mixture. However the soda lime filter material disclosed has a finite life, and if it is not changed when the life of the filter material is exhausted, the system would allow the nitrogen dioxide to be delivered to the biological system, and this could result in harm to the biological system.
In EP 0719159 (Jacobson), the problem of high energy arc discharges eroding the electrodes was addressed by disclosing a method of controlling the current across the air gap to a low level to produce a “glow discharge” to produce nitric oxide. The invention described multiple ways of initiating the glow discharge such as; using a separate high voltage spark circuit, reducing the pressure in the chamber or initially bringing the two electrodes closer together to initiate the spark. Once the glow discharge was established it was continuously maintained. To control the nitric oxide concentration to be delivered to the biological system the nitric oxide output was diluted with additional gas flow. The disadvantages of this approach are that there is a limited range of current that allows a glow discharge to be formed, and once formed the glow discharge needs to be continuously maintained. This limits the controllable range of the nitric oxide that can be produced. The required nitric oxide concentration to the biological system is achieved through diluting the nitric oxide flow from the reactor chamber with an additional diluting gas flow. However, this means both the gas flow rate and nitric oxide concentration cannot be controlled independently. If lower concentrations of nitric oxide at low gas flow rates are required, then a large portion of the nitric oxide generated is discarded resulting in low efficiency of operation and an additional apparatus associated with safely disposing of the unused nitric oxide gas flow.
U.S. Pat. Nos. 6,296,827 & 6,955,790 (Castor, et al.) disclose an alternative approach to avoiding electrode wear due to high energy arc discharges. These patents disclose an apparatus where a dielectric barrier material covers one of the electrodes and a corona discharge is produced in high frequency discharge pulses to avoid electrode wear. The reactor chamber has to be operated between 400° C. and 800° C. so the non-thermal plasma generates nitric oxide instead of nitrogen dioxide (NO2). The device also discloses the use of a catalyst operated at an elevated, temperature to convert any NO2 formed to nitric oxide. The temperature in the reactor is kept below 800° C. to avoid electrode erosion caused by oxygen radicals and above 400° C. to avoid NO2 being formed instead of nitric oxide. The apparatus disclosed that the nitric oxide gas flow is diluted by additional gas to produce the desired concentration of nitric oxide that is needed clinically. The disadvantage of this apparatus is that it requires extra electrical power to heat the gas to 400° C. to 800° C. and then requires the gas to be actively cooled after the reactor chamber before it can be used clinically. This significantly increases the power and complexity of the apparatus. It also has the same problem as EP 0719159, in that it provides a limited controllable range of nitric oxide being produced, and also relies on gas dilution to produce the desired nitric oxide concentration. Therefore it has the same limitation as described in EP 0719159 in that in some applications, it will result in unwanted nitric oxide gas flow that will need to be discarded resulting in inefficient operation and the need for additional apparatus for the safe disposal of the unused nitric oxide.
U.S. Pat. No. 7,498,000 (Pekshev, et al.) discloses a device for forming a nitric oxide containing gas flow using a continuous stationary DC arc discharge. The arc discharge is maintained at a constant voltage level of approximately 120V at 2.3 A, which maintains an arc temperature of 3500° K to 4000° K. The gas flow is then quenched in a water ethanol cooled chamber where it is rapidly cooled to approximately 1000° K to fix the nitric oxide that was generated in the arc discharge, it then goes on to a further cooling area where it is cooled to a temperature of 150° C. before it exits the outlet. The arc discharge is initiated with a high voltage spark discharge from a 5 kV circuit, and uses a stabilization electrode to maintain the are discharge. The nitric oxide gas concentration at the apparatus outlet is shown in FIG. 14 as 4,000 ppm nitric oxide, and the concentration is shown to decrease as a function of the distance from the outlet, dropping to about 500 ppm nitric oxide at a distance of 200 mm. This drop in nitric oxide concentration is due to the gas mixing with ambient air, and means a large part of the nitric oxide generated never gets to the intended biological target. It also means the user has to be very careful about the distance of the apparatus outlet to the biological target so the intended nitric oxide concentration is delivered correctly. The apparatus disclosed has the same problems as previous art in that the amount of nitric oxide generated is not well controlled and relics on wasteful dilution of the nitric oxide prior to delivery to the biological target. In addition there is no effort made to remove harmful NO2 that will also be formed in the are discharge.
The prior art have the following disadvantages:
1) The prior art does not disclose apparatus that can control the amount of nitric oxide over a wide range of gas flows and nitric oxide concentrations so that the device can be used for multiple distinct dosing regimens depending on the target application.
2) The prior art does not disclose apparatus that allows for a wide range of nitric oxide outputs without requiring additional gas flow dilution. This results in excess nitric oxide generation that has to be safely disposed of, causing extra cost and complexity.
3) The prior art that uses high voltage electric arc discharges to generate nitric oxide has high electrode wear due to electrode vaporization caused by the intense electric arc discharges.
4) The prior art that uses corona discharges require high reaction chamber temperatures to be maintained that needs additional power, and then require gas cooling systems to bring the gas flow back down to acceptable temperature levels prior to administration, thus adding to the cost, complexity and poor efficiency of the system.
5) None of the prior art provides a simple way of monitoring the correct functioning of the arc discharge so the amount of nitric oxide generated can be accurately predicted.
6) None of the prior art discloses a consumable filter for removing NO2 and other adulterants from the nitric oxide gas flow that when consumed, can provide the nitric oxide apparatus with a means of alerting the user that it needs to be replaced.